A United Nations working group is believed to have decided that WikiLeaks founder Julian Assange is being unlawfully detained.
Mr Assange has been living in the Ecuadorian embassy in London for more than three years and has been granted political asylum by the Ecuador government.
He believes he will be transported to the United States to be quizzed over the activities of WikiLeaks if he is extradited to Sweden. There is an espionage case against him in the US.
He filed a complaint against Sweden and the UK in September 2014 which has been considered by the UN Working Group on Arbitrary Detention.
The group of legal experts has made previous rulings on whether imprisonment or detention is lawful, which have led to people being released.
The decision is due to be published on Friday but it is understood that it has ruled in Mr Assange's favour.
It is expected the move will lead to calls from the UN for the UK and Sweden to release him.
Mr Assange said he will hand himself over to police for arrest on Friday if the UN group rules that he has not been unlawfully detained.
The Metropolitan Police have said they will make "every effort" to arrest the WikiLeaks founder should he leave the embassy.
Mr Assange is wanted for questioning in Sweden over one allegation of sexual assault, which he has always denied, and is fighting against extradition.
He was granted political asylum by Ecuador and has remained in their embassy since 2012.
In a statement published by the WikiLeaks activist group, Mr Assange said he expected to walk free if British and Swedish authorities do not receive approval from the UN group.
Mr Assange said: "Should the UN (working party) announce tomorrow that I have lost my case against the United Kingdom and Sweden, I shall exit the embassy at noon on Friday to accept arrest by British police as there is no meaningful prospect of further appeal.
"However, should I prevail and the state parties be found to have acted unlawfully, I expect the immediate return of my passport and the termination of further attempts to arrest me."
The statement was signed: "Julian Assange, Embassy of Ecuador, London."
According to the website justice4assange.com, the 44-year-old Australian has so far spent 1,885 days "under house arrest".
The embassy building remains under covert surveillance.
A Metropolitan Police spokesman said: "The operation to arrest Julian Assange does however continue and should he leave the embassy the MPS will make every effort to arrest him."
Sources at WikiLeaks said they were waiting for a formal announcement from the UN group.